Very high CPU usage running growisofs

Hi,
On 1.5.4-Preview using an up to date pkgsrc the growisofs program
is eating CPU (it's CPU bound on an AMD64 and burning very slowly ~ 1.5MB/s
instead of ~10MB/s which is usual for the media I have to hand). Pointing
truss at the running process reveals over 10k calls per second to
sys_set_tls_area - all identical viz:
sys_set_tls_area(0x0,0x28262f98,0x8) = 123 (0x7b)
Any ideas or suggestions for further investigation ?
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